1. Field of the Invention
The present invention relates to a view finder, and an optical apparatus incorporating the finder. More particularly, the invention relates to an external finder provided separately from a photo-taking optical system, in which a preferable finder image can be observed by appropriately designing lens structures of its objective optical system, its eyepiece optical system, and the like. The finder of the present invention is preferably usable in optical apparatuses, such as a still camera, a video camera, a digital camera, and a film camera.
2. Related Background Art
In a conventional camera with separately constructed photo-taking optical system and finder system, the finder system is also constructed as a zoom system when the photo-taking system is a zoom system, and a field magnification of the finder is changed in conformity with zooming of the photo-taking system. Generally, the structure of the zoom finder is required to be small in size and readily attain a desired zoom ratio since it is incorporated into a camera.
The Assignee of the present invention proposed the following real image type zoom finders in Japanese Patent Laid-Open Nos. 61(1986)-156018 (its U.S. counterpart is U.S. Pat. No. 4,842,395) and 1(1989)-116616, for example. In those zoom finders, its objective lens system is composed of plural lens units, zooming is performed by changing air spaces between the lens units, an object image with varying magnification formed by the objective lens system is converted to an erecting image by an image inverting member, such as a Porro prism, and the erecting image is observed by an eyepiece optical system.
In recent lens-shutter cameras, the zoom ratio of a photo-taking optical system tends to increase. Further, the size of a camera body is required to be small even when the zoom ratio of the photo-taking optical system is increased. Accordingly, the zoom ratio of a finder installed in the camera is also required to increase, and its size is desired to be small.
As a finder for achieving both of high zoom ratio and small size, there have been proposed finders with an objective optical system composed of plural lens units of positive power, negative power and positive power.
In Japanese Patent Laid-Open Nos. 2(1990)-173713 (its U.S. counterparts are U.S. Pat. Nos. 4,992,809, 5,095,326, 5,193,030, 5,225,927, and R.E.35,600), 8(1996)-43885 (its U.S. counterpart is U.S. Pat. No. 5,694,244), for example, the objective optical system includes, in the order from the object side, lens units of positive power, negative power and positive power, second and third units are moved during zooming. Further, in Japanese Patent Laid-Open No. 8(1996)-122857 (its U.S. counterpart is U.S. Pat. No. 5,748,381), the objective optical system includes, in the order from the object side, lens units of positive power, negative power, positive power and positive power, zooming is performed by the second unit, diopter accompanying the zooming is corrected by movement of the fourth unit, and the third unit remains stationary. Furthermore, in Japanese Patent Laid-Open Nos. 8(1996)-122856 and 11(1999)-281889, for example, the Assignee of the present invention proposed a zoom finder including an objective optical system composed of lens units of positive power, negative power and positive power in the order from the object side.
In the above-discussed structures in which two lens units are moved during the zooming, one of the two movable units performs most part of the zooming operation while the other moves to correct variation of the diopter accompanying the zooming.
In those zoom finders, when the zoom ratio is intended to be increased, amounts of movement of the two movable lens units need to be increased, leading to an increase in size of the entire lens system. Further, when the zoom ratio is increased while the size of the entire lens system remains small, the power of each lens unit only needs to be strengthened. However, if the power is simply strengthened, variation of aberrations during the zooming increase. Particularly, correction of aberrations of out-of-axis light rays becomes difficult. Accordingly, it becomes difficult to observe a preferable finder image over an overall zoom range.
It is an object of the present invention to provide a compact real image type finder in which a preferable finder image can be observed over an overall zoom range due to an appropriately-designed lens construction of an objective lens system including a zooming portion, and an optical apparatus incorporating the finder.
In one aspect, a finder of the present invention includes an objective optical system which includes, from an object side to an image side, a first lens unit having positive power, a second lens unit having negative power, a third lens unit having positive power, and a fourth lens unit having positive power; and an eyepiece optical system; wherein, when zooming is performed from a wide-angle end to a telephoto end, the second lens unit is moved toward the image side, the third lens unit is moved toward the object side, and the fourth lens unit is moved such that an air space between the fourth lens unit and the third lens unit is changed.
In another aspect, a camera of the present invention includes a photo-taking optical system, and a finder of the present invention whose optical axis is different from an optical axis of the photo-taking optical system.
These and further aspects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof in conjunction with the accompanying drawings.